Chain-type door latch and alarm

ABSTRACT

A door latch and alarm apparatus has a support mounted on the door and a support mounted on the doorjamb. A relatively inextensible and flexible element is anchored to the door support and has a free end provided with an end piece fittable in a latch on the doorjamb support. A pair of conductors, one of which may be the inextensible and flexible element, extend from the door support to the end piece where they are connected together via a resistor. At their other ends these conductors are connected to a resistance detector which operates an alarm whenever the resistance between the two conductors increases, as when the conductors are broken, and whenever it decreases, as when they are short-circuited.

FIELD OF THE INVENTION

The present invention relates to a door latch and alarm apparatus. Moreparticularly this invention concerns such an apparatus of the chain-typewhich allows the protected door to be opened only a limited distancefrom the doorjamb before preventing further opening.

BACKGROUND OF THE INVENTION

A chain-type door latch is known which has a support mounted on thedoor, another support secured to the doorjamb, and a flexible andinextensible element--normally a chain--anchored to the door support andreleaseably engageable with the support on the doorjamb. Such anarrangement allows the door to be opened a limited distance so that aperson on the inside of the door can see, talk to, and even receivesmall items from a person outside the door without having to open thedoor completely and allow the outside person in.

In a common variant on this type of latch a key-operable mechanism isprovided on the doorjamb support so that after opening the door by itsmain lock a person equipped with a key can reach in through the narrowgap that the chain allows the door to open to actuate the key mechanismon the doorjamb and thereby release the end piece of the chain. Such alatch is typically added to a door to increase the security thereof, inparticular in an apartment where the tenant fears that a key to the mainapartment-door lock might have fallen into the wrong hands.

Finally it is known to provide chain-type latches of the twoabove-described types with an alarm which sounds whenever an excessivestrain is exerted on the chain or other element extending between thedoor-mounted support and the doorjamb-mounted support. This arrangementhas the advantage that if the latch is forced the alarm will sound toalert persons in nearby areas and if possible to scare away the personmaking the forced entry.

This last type of described system has the disadvantage, however, thatthe latch can be overcome without sounding the alarm through the simpleexpedient of cutting the flexible element extending between the door andthe doorjamb, so that no strain is exerted on the chain or other elementthat would cause the alarm to be emitted. Another disadvantage of thesesystems is that during normal use an excessive force is sometimesexerted against the door, so that the alarm is emitted accidentally whenin fact no cause for alarm exists.

OBJECTS OF THE INVENTION

It is therefore an object of the instant invention to provide animproved door latch and alarm apparatus.

Another object is to provide such an apparatus which will emit an alarmwhenever an attempt is made to force the door open, or whenever thestructure preventing opening of the door is tampered with in an attemptto prevent the alarm from being emitted.

Yet another object is to provide such an apparatus which will emit analarm whenever it is subject to an excess force of tampering, but whichwill never emit an alarm under normal use conditions.

SUMMARY OF THE INVENTION

These objects are attained according to the instant invention in anapparatus of the above-described general type which is provided withcircuit means including at least one conductor extending between thedoor support and the end piece of the flexible and inextensible elementanchored in the door support and having a free end provided with thisend piece. This conductor forms a closed circuit path between the endpiece and the door support and is connected to alarm means which emitsan alarm whenever this path is interrupted. Thus with a system accordingto the instant invention if the conductors of the circuit means are cutor torn loose at either end, the alarm will be activated.

According to further features of this invention the circuit means has apair of conductors which are connected together at the end piece bymeans of a resistor. The alarm means includes a sensor which activatesthe alarm whenever resistance between the two conductors drops below apredetermined limit or rises above another predetermined limit. Thus ifthe cable is cut, so as to open-circuit the conductors, the alarm willgo off and similarly, if the conductors are exposed and shorted outbefore being cut by a more devious would-be entrant, the alarm will alsogo off. These conductors are relatively strong and durable between theend piece and the door support, but are made relatively frangible at theend piece, so that a strong pull exerted on the conductors will tearthem loose at the end piece and open-circuit them.

In accordance with another feature of this invention the relativelyfrangible ends of the conductors are formed as thin metallic stripsinside the end piece which are overlain with spacing by the conductivehead of a screw which holds the doorjamb support to the doorjamb so thatwhen a strong tug is exerted on these conductors it will pull up andshort against the screw head. Thus if a continuously growing force isapplied to the conductors they will first of all short out, therebyactivating the alarm, and thereafter open-circuit, which will alsoactivate the alarm in the event that they do not make good contact withthe head of the screw that shorts them out. It is also possible toanchor the two conductors one one of the supports to a highly fragileanchor so that any strong pull on these conductors will break them freeand open-circuit them so as to set off the alarm.

The conductors according to this invention may be provided in arelatively light cable extending parallel to the flexible andinextensible element between the door support and the doorjamb support.Such a cable would have a length slightly greater than that of theflexible and inextensible element, normally constituted as a chain, sothat it would not be pulled taut unless the chain were broken. It isalso possible to use the flexible and inextensible element itself as oneof the conductors, or to constitute it as a heavy wire inside amultiwire cable that forms both the conductors and the flexible andinextensible element.

The latch according to this invention may be a key-operated mechanismwhich can be operated through the gap between the door and the doorjambonce the main door lock has been operated and the door opened to theextent allowed by the flexible and inextensible element. The latch is,therefore, operated by the key to release the end piece to which isattached the flexible element and the conductors.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a perspective view of the apparatus according to thisinvention;

FIG. 2 is a schematic diagram of the apparatus of FIG. 1;

FIGS. 3 and 4 are large-scale perspective and end views of a detail ofthe apparatus of FIG. 1;

FIG. 5 is a view similar to FIG. 1 of another apparatus in accordancewith this invention; and

FIGS. 6, 7 and 8 are schematic diagrams illustrating alternative circuitarrangements for the apparatus of FIG. 5.

SPECIFIC DESCRIPTION

Shown in FIG. 1 a latch and alarm apparatus is adapted to be mounted ona door 1 normally received in a doorjamb 2. The apparatus comprises adoor-mounted support or housing 3 and a doorjamb-mounted support 4. Akey-operated latch 5 on the doorjamb support 4 is lockingly engageablewith an end piece 7 carried at the free end of a chain 6 and of atwo-conductor light cable 8. The chain 6 is somewhat shorter than thecable 8, to allow the door 1 to be opened to a limited extent so that itis spaced by gap 9 from the jamb 2.

As shown in FIG. 2 the chain 6 is connected at one end to an anchorscrew 10 on the end piece 7 and at its other end to an anchor screw 13on the housing 3. The cable 8 has two conductors 8a and 8b which areeach connected in the end piece 7 to one end of a respective conductorsstrip 11. The other ends of these strips 11 are interconnected by meansof a resistor 12. The conductors 8a and 8b are connected at their otherends on the housing 3 to a terminal block 14 in turn connected to aresistance detector 26 itself connected to an acoustical alarm or horn27.

The doorjamb support 4 as shown in FIGS. 1, 3, and 4 is shaped as achannel having a pair of sides or cheeks 16 and a base plate 17. Upperprojections 18 of the base plate 17 equispaced between the cheeks 16 areformed with throughgoing holes through which pass the shanks of screws19 that secure the support 4 to the doorjamb 2. The screws 19 have largeheads and the end piece 7, which fits plug-fashion into the bottom ofthe support 4, is of U-section with the two strips 11 positioned sothat, when the end piece 7 is inserted fully upwardly into the supportor socket 4, the strips 11 will underlie the heads of the screws 19. Astop 15 at the upper end of the channel forming the socket or guide 4determines the end rest position for the endpiece 7. In this position abolt operated by the key mechanism 5 fits into a recess on the upperwall of the U-section endpiece 7 to lock it tightly in place. Rotationof the key in the key mechanism 5 pulls the bolt out of the recess andallows the endpiece 7 to drop downwardly out of the support 4.

The user of such a lock need merely push the end piece 7 upwardly intothe support 4 to set it, which action can be done easily from inside thedoor after closing it and just as easily from outside the door throughthe gap 9. To release the endpiece 7 from the support 4 the key isrotated in latch 5.

A person attempting to make a forced entrance through the door 1 fromthe outside will, after having overcome the main door lock, have toovercome the latch and alarm apparatus according to this invention. Ifsuch a would-be entrant simply exerts a sufficiently large force againstthe door to break the chain 6, the force will be invariably sufficientto exert considerable tension on the cable 8. Such tension will eitherpull the conductors 11 against the head of the screw 19 so as to shortthem out, whereupon the resistance detector 26 will set off the alarm27, or will simply exert so much tension on these relatively fragilestrips 11 that they will rupture, thereby open-circuiting the conductors8a and 8b, so that once again the resistance detector 26 will set offthe alarm 27. If in the alternative, the would-be entrant cuts the cable8 the conductors 8a and 8b will again be open-circuited so that thealarm 27 will sound. A knowledgeable burglar might reach through the gap9 and cut open the cable 8 so as to expose the conductors 8a and 8b.These conductors 8a and 8b could then be connected together in themanner that is standard to overcome many conventional burglar alarms, sothat the cable 8 could be cut downstream of the location where theconductors 8a and 8b are shorted. Such a procedure will not, however,work with the system according to this invention since this will dropthe resistance between the conductors 8a and 8b to below the thresholdlevel for the resistance detector 26, which will once again set off thealarm 27.

In the arrangement of FIG. 5 the use of a separate flexible andinextensible element 6 is eliminated and instead this element or chain 6and the cable 8 are replaced by a single cable 20 secured at a clip 21to the door 1 below the housing 3. This cable 20 as shown in FIG. 6 mayhave two conductors 20a 20b functionally indentical to the conductors 8aand 8b, and a third conductor 22 which is considerably larger andstronger and which fulfills the function of the chain 6 of FIG. 1.

In the arrangement of FIG. 7 a cable 20' has a light-gauge conductor20a' and a heavy-gauge conductor 22', the latter serving the function ofthe chain 6 of FIG. 1 and also serving as one leg of the conductor pathto and from the end piece 7.

The arrangement of FIG. 8 uses a cable 20" having a conductor 20a" and20b". This cable 20" is relatively heavy and is received at a cableclamp 23 on the end piece 7 and at a similar such cable clamp 24 on thehousing 3. The conductor 20b" is connected to circuitry of thearrangement at a weak end region 25 of the printedcircuit board for theresistance detector 26. Thus during normal use the clamps 23 and 24 willbe sufficient to withstand the relatively small tensile stresses appliedto the cable 20". If too much force is exerted the clip 24 will normallyrelease so that the conductor 20b" will exert a considerable pull on thefragile region 45 and will break loose the head of the mounting screw,thereby open-circuiting the arrangement and setting off the alarm 27. Ifthis clip 24 does not release before the clip 23 the device willfunction either by shorting out or ripping off the strip 11 as describedabove with reference FIGS. 1-4.

It is of course within the scope of this invention to provide strips 11and screws 19 on the housing 3 as well as on the end piece 7.Furthermore the alarm unit itself could be placed on the doorjamb ratherthan on the door, so that the resistor 12 would be on the door and thealarm 27 and operating circuit 26 on the doorjamb.

I claim:
 1. A door latch and alarm apparatus comprising:a door-mountedsupport; a doorjamb-mounted support; a relatively inextensible andflexible element anchored to said door support and having a free endprovided with an end piece receivable in said doorjamb support; latchmeans including key-operable mechanism on said doorjamb support forreleasably securing said end piece thereto, whereby the door can open alimited distance from the doorjamb before said element becomes taut andimpedes further opening; circuit means including a pair of conductorsextending between said door support and said end piece and a relativelyfragile connector in said end piece forming a closed conductive pathwith said conductors, said connector being so much weaker than saidelement that when same is stressed beyond a certain limit said connectorinterrupts said path before said element breaks; and alarm means on saiddoor-support connectable to said conductors for emitting an alarm whensaid path is interrupted.
 2. The apparatus defined in claim 1 whereinsaid circuit means includes a resistor connected between said conductorsat said end piece, said alarm means including means for detecting theresistance between said conductors and for emitting said alarm when saidresistance increases or decreases substantially.
 3. The aparatus definedin claim 1 wherein said circuit means includes a cable separate fromsaid element and having said pair of conductors.
 4. The apparatusdefined in claim 3 wherein said element is a chain.
 5. The apparatusdefined in claim 3 wherein said element is shorter between said doorsupport and said end piece than said cable.
 6. The apparatus defined inclaim 1, a cable extending between said door support and said end pieceand having a plurality of wires constituting said element and saidconductors.
 7. The apparatus defined in claim 6 wherein said cable hastwo such wires, one constituting said element and one of said conductorsand the other constituting the other conductor.
 8. The apparatus definedin claim 6 wherein said cable has three such wires, one constitutingsaid element and the other two constituting said conductors.
 9. Theapparatus defined in claim 1 wherein said conductors are relativelystrong between said end piece and said door support and relativelyfragile in said end piece.
 10. The apparatus defined in claim 9 whereineach of said conductors is formed in said end piece as a fragileconductive strip constituting said connector.
 11. The apparatus definedin claim 10 wherein said doorjamb support includes at least one screwsecuring said doorjamb support to said doorjamb and having a shankextending between and past said strips and a head overlying andoverlapping said strips, whereby said strips can be pulled intoshort-circuiting electrical contact with said head.
 12. The apparatusdefined in claim 10 wherein said strips each have one end connected tothe respective conductor and another end connected to the other strip.13. The apparatus defined in claim 1 wherein one of said supports isprovided with a relatively fragile anchor for said conductors and theother of said supports is provided with a more substantial anchor forsaid conductors.